In the field of long-distance optical communication, as the performance of optical communication systems has increased, management and control of the wavelength dispersion characteristics of optical fibers have become important issues.
Generally, optical fibers have a negative group velocity dispersion (GVD) in wavelength bands used for optical communication (for example, the 1.55 μm band). The GVD is one reason for the deterioration of the transmission characteristics of optical-pulse signals. Therefore, it is necessary to control and compensate for the dispersion characteristics of optical transmission paths, and it is very important to measure the total amount of dispersion of optical fibers.
One GVD measuring technique uses a variable-wavelength CW light source or pulsed light source to measure the wavelength dependency of the optical propagation time. In this technique, multiple-point measurement is employed with the use of wavelength as a parameter. This measurement takes a long time, and therefore it is difficult to control the GVD characteristics in real time.
The present inventors have previously proposed a new GVD measuring method which uses an optical frequency domain reflectometry (OFDR) method (“Optical-Fiber Group-Velocity-Dispersion Measurement Using Frequency-Shifted Feedback Laser” by Masato Yoshida, Toshiyuki Miyamoto, Takefumi Hara, Koichiro Nakamura, and Hiromasa Ito, in Technical Report of the Institute of Electronics, Information and Communication Engineers, OCS 98-103 (1999), 25). The present inventors have also proposed a GVD measuring device using a frequency-shifted feedback laser (FSF laser) as a frequency-chirped light source (“Automatic Group-Velocity-Dispersion Measuring System using Frequency-Shifted Feedback Fiber Laser” by Toshiyuki Miyamoto, Masato Yoshida, Takefumi Hara, Koichiro Nakamura, and Hiromasa Ito, in the Proceedings of Spring Conference of the Institute of Electronics, Information and Communication Engineers, C-3 (1999)).
The GVD measuring method which the present inventors have previously proposed directly calculates the GVD value of an optical transmission path according to the change in optical chirp rate during light propagation, and one feature is the short time required for measurement.